Automated overlay metrology system

ABSTRACT

Non-imaging measurement is made of misalignment of lithographic exposures by illuminating periodic features of a mark formed by two lithographic exposures with broadband light and detecting an interference pattern at different wavelengths using a specular spectroscopic scatterometer including a wavelength dispersive detector. Misalignment can be discriminated by inspection of a spectral response curve and by comparison with stored spectral response curves that may be empirical data or derived by simulation. Determination of best fit to a stored spectral curve, preferably using an optimization technique can be used to quantify the detected misalignment. Such a measurement may be made on-line or in-line in a short time while avoiding tool induced shift, contact with the mark or use of a tool requiring high vacuum.

This application is a division of U. S. Pat. application Ser. No.10/704,979, filed Nov. 12, 2003, now U.S. Pat. No. 7,087,352 which is acontinuation of U.S. Pat. application 09/881,026, filed Jun. 15, 2001,now abandoned; both of which are assigned to the assignee of the presentinvention and fully incorporated by reference herein. Priority under 35U.S.C. § 120 of both applications is hereby claimed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to multiple sequentiallithography processes and, more particularly, to metrology techniquesfor measurement and characterization of overlay and alignment accuracyfor sequential lithographic exposures and in-line and on-linelithographic exposure, scanner or stepper tools.

2. Description of the Prior Art

Lithography processes are currently used in many research andmanufacturing environments. Among these environments, one of the moreeconomically important is that of semiconductor integrated circuitmanufacture. In this field, increased functionality, performance andpotential economy of manufacture has driven the development of numeroussuccessive generations of devices having minimum feature size regimes ofincreasingly small dimensions and correspondingly increased devicedensity. Currently, feature size regimes of one-quarter micron areavailable in commercial devices with significant further reductions ofminimum feature size foreseeable or in experimental stages ofdevelopment.

While sophisticated processes have been developed allowing production ofstructures much smaller than can be resolved by known lithographicprocesses, it can be understood that at least one lithographic processis required to establish the location and basic dimensions of variouselectronic devices (e.g. transistors, capacitors and the like) on thewafer and chip areas thereof. However, as a practical matter, numerouslithographic exposures and processes are generally required sinceformation of devices in different areas and different layers aregenerally included in current and foreseeable designs.

For example, at extremely small minimum feature size regimes, ifdifferent structures of a particular device, such as the gate andchannel of a field-effect transistor must be formed with differentlithographic processes, overlay accuracy must be maintained at a highlevel to avoid significantly altering electrical characteristics of thedevice. In much the same manner, interlayer connections (e.g. vias,studs and interconnect layer patterns) must be overlaid on each otherwith extreme accuracy in order to reliably and repeatably form lowresistance connections which will remain reliable when placed inservice. The accuracy of positioning in all of these circumstances willhereinafter be referred to as overlay accuracy.

It has been observed that integration density in successive generationsof integrated circuits roughly doubles every two to three years. Thisincrease in integration density corresponds to a reduction of minimumfeature size of about 30% over similar periods. The overlay accuracyrequirement must be held to a fraction (generally about one-third orless) of the minimum feature size to maintain device geometry andconnection reliability. To maintain such accuracy, overlay and stitchingmeasurement accuracy (mean value plus or minus 3 sigma) must bemaintained to a metrology error budget of about 10% or less of themaximum allowable overlay error or about 3% of the minimum feature size(less than 6 nm for 0.18 micron minimum feature size technology).

Commonly used metrology techniques are based upon optical microscopyobservation of overlay targets for quantifying the overlay error. Thesetechniques have relied upon a feature-in-feature imaging process where afeature such as a square or line is formed within another generallysimilar shape (e.g., box-in-box or line-in-line) to define the overlaytarget. The smallest feature size of the overlay target is typically ofthe order of 1 micron, so that it can be imaged with high contrast, wellwithin the resolution limit of a conventional optical microscope.

Measurement of these overlaid features with, for example, a scanningelectron microscope (SEM) of atomic force microscope (AFM) has allowedmeasurement data to be developed which can be processed to providemeasurement resolution somewhat greater than the resolution of availablelithography tools, even though the features measured are typicallylarger than the features which can be lithographically resolved.However, such measurement techniques require complex and expensive SEMor AFM tools which are inherently of low throughput and the measurementis necessarily destructive, decreasing manufacturing yield.

Perhaps more importantly, decreases in minimum feature size andincreases in integration density have required increasingly complex,expensive and difficult to use measurement tools while measurementsproduced are of reduced repeatability, reproducibility, tool inducedshift (which are the principal components of the metrology error budget)and quantitative certainty (e.g. confidence factor) as limits of bothlithographic and microscopic resolutions are approached, particularlywhen the imaged features measured are necessarily much larger than theminimum feature size. Further, it is not only necessary toquantitatively evaluate the positioning accuracy of overlaid or stitchedtogether features, but the profile of the exposed and developed resistand/or lithographically produced structures must also be evaluated inseparate measurements in order to assure that structures with thedesired electrical properties are produced.

Thus, it is seen that, at the present state of the art, known overlaymeasurement techniques can only be extended to smaller regimes offeature size at relatively great tool expense and process difficulty andcomplexity and increasing uncertainty and decreasing repeatability ofresult. Further, it is not at all clear that advances in microscopyprocesses or other inspection devices which rely upon imaging offeatures will be able to support manufacturing processes of foreseeableregimes of integrated circuit feature size and integration density.

Spectroscopic reflectometry and spectroscopic ellipsometry are known andwell-understood techniques for making quantitative observations ofsurfaces and structures but have only rarely been applied tolithographic processes or characterization of the performance oflithography tools. However, one such application is the use of specularscatterometry to provide a non-destructive measurement of profiles ofresist grating patterns of high resolution. This was presented by Spanosand his students (X. Niu, N. Jakatdar) at the First Small FeatureReproducibility workshop (UC SMART Program Review, of which the assigneeof the present invention is a funding participant), held at theUniversity of California on Nov. 18, 1998. Spanos outlined plans to usea spectroscopic ellipsometry sensor to extract profiles of 180 nm and150 nm linewidth resist features. FIG. 8, derived from that presentationshows spectral response curves of Log(Tan Ψ), the amplitude ratio ofcomplex 0th order TE and TM reflectivities, for two sets of nominallinewidth features (250 nm and 100 nm) with ±7% variation linewidths.The result for the 100 nm feature shows a ±7% linewidth variation on a100 nm nominal feature (or 7 nm) produces differing spectral curves thatcan be used to measure linewidth. However, it should be recognized thatthe curves are quite similar in shape since they are produced byregularly spaced features, and that both the shape of the spectralcurves and the location of the various peaks and valleys can be used toevaluate the linewidth corresponding to a measured spectral curve, bybest fit comparison to a library of spectral curves, previouslygenerated by simulation and verified by comparison to calibrationexperimental data.

Specular spectroscopic ellipsometry measures the 0th order diffractionresponses of a grating at multiple wavelengths and fixed incidence angleusing a spectroscopic ellipsometer sensor. A description of themeasurement technique can be found in the publication “SpecularSpectroscopic Scatterometry in DUV Lithography” by X. Niu et al. (SPIE,Vol. 3677, pp. 159-168, March, 1999. The ellipsometer sensor measurescomplex reflectivity for two orthogonal light polarizations.

SUMMARY OF THE INVENTION

The meritorious effects of the invention include provision of an opticalmetrology technique which does not rely upon imaging of features forinspection, increased resolution and quantitative accuracy andrepeatability which can be performed with apparatus of much reducedexpense and complexity at greatly increased throughput, and simultaneousand non destructive overlay position and feature profile measurements.

In order to obtain these effects, a method and apparatus are providiedwhich perform non-imaging metrology apparatus comprising storage ofspectral curves, measurement with a specular spectroscopic scatterometerof reflection from a plurality of marks formed by two lithographicexposures and forming a periodic structure, and providing comparison ofprocessed signals output from said specular spectroscopic scatterometerwith said spectral curves to evaluate misalignment of said twolithographic exposures.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1A, 1B and 1C are illustrations of a box-in-box image used inknown metrology techniques,

FIG. 2 shows respective feature levels in accordance with a preferredform of the invention,

FIG. 2A shows the images of the features of FIG. 2 overlaid in desiredalignment,

FIG. 2B shows the images of the features of FIG. 2 overlaid withmisalignment,

FIG. 3 shows a simulated Fourier spectrum developed in accordance withthe invention and corresponding to FIG. 2A (correct alignment)

FIG. 4 shows a simulated Fourier spectrum developed in accordance withthe invention and corresponding to FIG. 2 b (misalignment),

FIG. 5 shows a simulated Fourier phase spectrum developed in accordancewith the invention with aligned and misaligned responses in accordancewith the invention overlaid on each other,

FIG. 6A is a schematic diagram of the measurement apparatus inaccordance with the invention, including a spectroscopic reflectometersensor and detector and a wafer with composite overlay targets withexact alignment,

FIG. 6B is a schematic diagram of the measurement apparatus inaccordance with the invention, including a spectroscopic reflectometersensor and detector and a wafer with composite overlay targets with somemisalignment,

FIG. 7 is a schematic illustration of the measurement process inaccordance with the invention, and

FIG. 8 is an illustrative chart from a presentation by Spanos, Niu andJakatdar at the First Small Feature Reproducability workshop.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIGS. 1A, 1B and1C, there is shown a typical box-in-box metrology feature exemplary offeatures for such purposes known and used in the art. The box-in-boxfeature 16 shown in FIG. 1C is a composite feature formed by twooverlaid lithographic exposures corresponding respectively to features12 and 14 of FIGS. 1A and 1B, respectively, which are generallysequentially performed and each preceded by deposition of a layer ofresist and followed by development of resist layer and possiblyincluding etching or material deposition processes between thelithographic exposures. One feature will be larger than the other andthe smaller feature should be of dimensions which, ideally, closelyapproach the minimum feature size of interest.

At the present state of the art, however, the smaller and largerfeatures are generally produced with a transverse dimension of the orderof a few microns. Such dimensions are about an order of magnitude ormore larger than minimum feature size of the current generation ofcommercially available integrated circuits. Such a difference in minimumfeature size puts stringent demand on processing of measurement data tohold the overlay accuracy budget within a small fraction of the minimumfeature size and may provide profile shapes which are not representativeof the profiles of much smaller features. The relative positions ofthese features and their material profiles must then be observed byoptical microscopy, SEM or AFM in separate processes and the resultingdata processed. All of these processes are imaging techniques and allhave serious limitations. The optical microscopy method is limited inimage resolution. The AFM method is a quasi-contact technique and isvery slow. The SEM method requires that observation be performed in ahigh vacuum and transfer of the wafer and pumping an expensive vacuumchamber down to an appropriate pressure greatly extends the amount oftime required for measurement to be made; which is, itself, ofsignificant duration of about ten seconds or more per measurement. Itmay be required for the wafer to be sectioned and illuminated atdifferent angles requiring different set-up for different measurements.In any case, measurement is destructive, indirect and of necessarily lowthroughput while requiring apparatus and process methodology of high(and increasing) complexity and cost as well as substantial processingof the raw measurement data.

FIG. 2 shows two levels 20, 22 of exemplary features in accordance withthe present invention. In theory, any series of repeated shapes withintervening repeated shapes in another level could be used in accordancewith the basic principles of the invention. However, arrays of lines(which can be relatively short) are preferred to minimize dataprocessing complexity and time. Further, while it is contemplated thatthe invention would preferably be practiced using test patterns similarto those of FIG. 2 and in which one level (sequentially second) wasformed of resist, longer lines may provide enhanced signal-to-noiseratio or other improvement in raw data and it should be understood thatthe invention can be practiced with completed structures such asrelatively long parallel connections (and which may include angledportions). It is preferred that the two arrays of features be similarlyand regularly spaced (e.g. of constant pitch/spacing) and of the samewidth but different widths and spacing can be accommodated and,moreover, accurately evaluated by the practice of the invention.

The exposures of the respective level patterns of FIG. 2 can be made ina manner which is fortuitously aligned, as shown at 24 in FIG. 2A, butwill generally be misaligned to some degree, as shown at 26 in FIG. 2B.In accordance with the principles of the invention, the compositepattern in accordance with, for example, FIG. 2A or FIG. 2B willfunction as a diffraction grating and the respective features need notbe specifically imaged in the course of the measurement process. By thesame token and in accordance with the invention, restrictions on theillumination source by which the test pattern is observed are largelyremoved, greatly simplifying the required measurement apparatus and theprocess of its use while increasing throughput of the measurementprocess.

The theory and operation of a diffraction grating are well-understoodand software exists which allows analysis and simulation of its effectswhich are extremely sensitive to geometry and spacing of the elementswhich comprise the periodic structure of a diffraction grating.Specifically, when a diffraction grating is illuminated at a known anglewith either monochromatic (coherent or non-coherent) or broadband light,an interference pattern is developed by differences in opacity orreflectivity of respective areas therein. Depending on the geometry andspacing of the marks forming the diffraction grating, the interferencepattern will exhibit specific and characteristic behaviors due to theinteraction of the wavelengths of the illuminating radiation (e.g.light) and the spacing and other geometry of the marks which causesreflected or transmitted light at certain angles and wavelengths to beeither reinforced or cancelled. Similar effects are observable withrespect to phase and polarization of the transmitted or reflectedradiation.

Measurement of amplitude and phase are generally referred to asspectroscopic reflectometry. Measurement of amplitude, phase andpolarization are generally referred to as spectroscopic ellipsometry.Either may be used in the practice of the invention; the latterproviding somewhat more detailed characterization of spacing andmaterial profile but reflects additional “degrees of freedom” in thecaptured data. If spectroscopic reflectometry provides sufficientaccuracy in determining misalignment error, then it is preferred foroverlay metrology since less complex processing and/or matching of datais required. This would be the case when the overlay patterns have nearideal profiles of “square” profiles. In this case, it is necessary tomodel a large number of possible variations of the composite overlaystructure and to build up a library of response curves which can becompared and matched to the measurement data.

However, if the profiles of the overlay patterns are rounded ordistorted, the ellipsometry with its additional degrees of freedom canprovide better accuracy in determining the misalignment error. In thiscase, it is necessary to model an even larger number of possiblevariations of the composite overlay structure and to build up a libraryof response curves which can be compared and matched to the measurementdata. This library of response curves needs to be generated for each newsituation of level-to-level overlay, for different degrees ofmisalignment error, profile distortion and using the measured opticalconstants of the substrate and layer materials. Fortunately, the waferprocesses are very well characterized so that once the library isgenerated for a new situation, the misalignment error can be determinedvery rapidly by comparing stored spectra with the measured one.

FIGS. 6A and 6B illustrate the measurement geometry and apparatus in thecase of reflecting aligned or misaligned overlay targets. Forsimplicity, a simple reflectometry sensor is shown. The measurementapparatus comprises a broadband light source producing a collimatedlight beam incident on the overlay target area at some fixed angle ofincidence. The reflected light is collected by a wavelength dispersivedetector that measures amplitude and phase of the reflected light acrossthe desired spectrum. In FIG. 6A, the alignment of the second mask levelto the first mask level is perfect and the composite overlay targetproduces a spectral response as shown. In FIG. 6B, the alignment isslightly off and a different spectral response is measured.

In both cases the spectral response is analyzed by a dedicated computer,compared to stored signals and the misalignment error, if any, isdetermined. Because the dimension of the individual overlay target canbe small (such as one micron or less), the accuracy of the determinationof the alignment error can be very high. Other advantages are speed(because signal processing is limited to comparison of the measuredspectral curve to stored data and determination of best fit or match),the avoidance of a need for imaging, freedom from tool induced shifterror and non-contact operation.

The optical spectroscopic reflectometry or ellipsometry sensor is verycompact and can therefore be incorporated in a process tool such as aresist track developer to provide on-line metrology capability where itcan provide direct feedback on the alignment system performance of thestepper. The same sensor could also be central to a stand-alone overlaymetrology tool for in-line metrology applications.

A simulation of the FFT amplitude variation of spectral reflectance datafrom the pattern of FIG. 2A is shown in FIG. 3 and a simulation of FFTamplitude variation of spectral reflectance data from the pattern ofFigure 2B is shown in FIG. 4. It is assumed for purposes of thisdiscussion that the differently shaded portions 22, 24 of FIG. 2A (and2B) have different reflectivity and that the marks include at least onemark which is of differing width. It is also assumed that illuminationis with broadband light and that the reflected light is analyzed with awavelength dispersive detector to provide a spectral curve ofreflectivity (amplitude and phase) as a function of wavelength. Theresulting curve will be similar in some respects to FIGS. 3 and 4 andcan be processed to obtain the same overlay alignment information, aswill be readily understood by those skilled in the art. The correctlyaligned marks of FIG. 2A are assumed to be of substantially constantpitch while two (or more) distinct pitches or spacings are exhibited bythe misaligned marks of FIG. 2B.

FIG. 3 shows a plurality of peaks of light amplitude at differentfrequencies or wavelengths (calibrated as a function of 1/pixel which isbasically equivalent to inverse wavelength but specifically related bythe calibration to multiples of lithographic tool resolution or minimumfeature size). Sharp peaks 32 and broad peaks 34 are evident and aredependent on incident geometry, reflectivity and profile of individuallines. In FIG. 3, both the sharp peaks and the broad peaks aresubstantially symmetrical while in FIG. 4, substantial asymmetry isevident, particularly in the broad peaks 42 and the sharp peaks 44 oflonger wavelength. This asymmetry of peaks in FIG. 4 is due to thedifferent spacings caused by misalignment in the composite pattern ofFIG. 2B but is substantially absent from FIG. 3 since the pitch of themarks is substantially constant. Thus, it is seen that the shape of thespectral curve is extremely sensitive to the existence of slightvariation in spacing of a periodic structure (which would includefeatures at a plurality of pitches or periodic spacings due to anymisalignment) and even small degrees of misalignment can bediscriminated by inspection and quantified by comparison with empiricalor simulated data.

FIG. 5 shows a simulation of phase variation with wavelength includingtwo traces. The solid trace 52 corresponds to the aligned overlaypattern of FIG. 2A while the dashed trace 54 corresponds to slightmisalignment. It should be noted that both the functional variation(e.g. trace shape) with wavelength and the magnitude of the variationvaries with the degree of misalignment and is thus, like amplitudevariation with wavelength of FIGS. 3 and 4, a very sensitivequantitative indicator of the degree of misalignment, after processing.

It is desirable to develop a quantitative measurement of overlaymisalignment so that correction can be made or a decision can be made asto whether or not overlay accuracy is sufficient for processing tocontinue. (If the measurement is made including a previously formedstructure and a developed pattern of resist, if the overlay misalignmentis unacceptably great, the developed resist can be stripped and anotherresist layer applied, exposed and developed; thus saving the previousmanufacturing expenditure of processing the wafer to that point. Thissavings may be substantial since many critical lithographic patterningprocesses are performed in making connections to devices after processesfor forming those devices are substantially complete.) While the degreeof asymmetry or phase function with overlay misalignment distance may beaffected by other parameters such as mark size and profile, particularlyif spectroscopic ellipsometry techniques are employed, the change inasymmetry or phase function with change in overlay misalignment can becharacterized and are incorporated in the library data.

Therefore, for a given feature size regime and with at least somesimilarity in feature geometry (e.g. pitch, width and profile) acalibration or verification of the process in accordance with theinvention may be achieved by exposing overlaid patterns as describedabove in connection with FIGS. 2, 2A and 2B with differing misalignmentsand making spectroscopic observations such as FIGS. 3 and 4, followed byprocessing of the spectral curves and comparison to stored curvesobtained from prior simulations to determine the misalignment errors Thesame patterns can then be observed or measured with SEM or AFM and theresults compared to the calculated misalignments. If the results do notagree, then it may be necessary to perform additional simulations tobetter model the composite overlay target physical properties using SEMcross-sectional data.

Referring now to FIG. 7, the methodology of the invention will now besummarized. Composite overlay targets 71 are obtained by superimpositionof two successive mask level patterns in a reserved area of of the wafer72, referred to as an overlay measurement mark area, The second masklevel is defined as a resist structure and the first mask level isdefined as an etched structure on the same area of the wafer substrate.

The wafer is brought under a specular spectroscopic scatterometer 73 ofeither reflectometer or ellipsometer type which is used to analyze thecomposite overlay target. Measurement data (and amplitude and phasespectral curves) collected by a wavelength dispersive detector 74 areprocessed by a data processor 75, such as a workstation computer sinceprocessing power demands may be high for running an optimization programfor matching measured data to stored data in a library 76 obtained byprior simulation with an exact model representation of the compositetarget structure for a full range of misalignment values. A globaloptimization technique is used to determine the best fit to modeled dataand to thus quantify the misalignment value for the target.

In view of the foregoing, it is seen that the invention provides atechnique of measuring overlay misalignment which does not requireimaging of overlaid features and is thus applicable to feature sizeswell below one micron. The invention provides improved resolution,repeatability, reproducibility and quantitative accuracy usingsimplified apparatus and procedures of reduced complexity and cost andwhich can be performed on-line or in-line, possibly concurrently withother measurements of interest. The invention utilizes an opticaltechnique but, since it does not require imaging and avoids the need fora microscope, it is free from tool induced shift error (which is ameasure of the impact of tool asymmetry on measurement error in animaging system). Further, the invention provides for quantitativemeasurement of misalignment determined from processing simple and directobservation of a spectroscopic response corresponding to a detectedinterference pattern using broadband illumination.

While the invention has been described in terms of a single preferredembodiment, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. A method comprising: superimposing a first mask level pattern and asecond mask level pattern in a predetermined area of a substrate toobtain a composite overlay target; and analyzing said composite overlaytarget using a specular spectroscopic scatterometer, comprising:illuminating said composite overlay target with a light source;collecting measurement data of a reflected light from said compositeoverlay target using a detector, and determining geometry of saidcomposite overlay target from said measurement data.
 2. The method ofclaim 1, further comprising: processing said measurement data using adata processor to generate measured spectral curves.
 3. The method ofclaim 2, further comprising: comparing said measured spectral curveswith stored spectral curves in a library to determine misalignment ofsaid first mask level pattern and said second mask level pattern.
 4. Themethod of claim 3, further comprising: simulating a model representationof said composite overlay target for a full range of misalignment valuesto obtain said stored spectral curves.
 5. The method of claim 4, furthercomprising: determining a best fit of said measured spectral curves tosaid model representation to quantify a misalignment value of saidcomposite overlay target.
 6. The method of claim 4, wherein saidsimulation comprise generating a spectral curve for each situation oflevel-to-level pattern.
 7. The method of claim 6, wherein said situationis one of degree of misalignment error, profile distortion and opticalconstant of the substrate.
 8. The method of claim 1, wherein the firstmask level comprises a first series of repeated shapes and the secondmask level comprises a second series of repeated shapes that intervenewith the first series of repeated shapes.
 9. The method of claim 8,wherein the first series of repeated shapes and the second series ofrepeated shapes are substantially similar.
 10. The method of claim 8,wherein the first series of repeated shapes comprises an array of lines.11. The method of claim 10, wherein the lines are at a constant pitch.12. The method of claim 10, wherein the lines have substantially thesame width.
 13. The method of claim 10, wherein the lines have varyingwidths.
 14. The method of claim 10, wherein the lines have varyingspacing.
 15. The method of claim 1, wherein the first mask level is anetched structure.
 16. The method of claim 1, wherein the second masklevel is a resist structure.
 17. The method of claim 1, wherein thespecular spectroscopic scatterometer comprises a reflectometer.
 18. Themethod of claim 1, wherein the specular spectroscopic scatterometercomprises an ellipsometer.
 19. The method of claim 1, wherein saidmeasurement data comprises at least one of amplitude, phase andpolarization measurement.
 20. The method of claim 1, wherein saidcomposite overlay target functions as a diffraction grating.
 21. Themethod of claim 1, wherein said light source is a broadband lightsource.
 22. The method of claim 1, further comprising: producing acollimated light beam incident on said predetermined area at a fixedangle of incidence.
 23. The method of claim 1, further comprising:collecting a reflected light from said substrate.
 24. The method ofclaim 23, wherein said measurement data comprises one of amplitude,phase and polarization of said reflected light across a predeterminedspectrum.
 25. The method of claim 1, wherein said detector is awavelength dispersive detector.
 26. A method of measuring overlaymisalignment without imaging overlaid features, said method comprisingsteps of: exposing misaligned overlaid features; making spectroscopicobservations; and calculating spectral curves from said spectroscopicobservations.
 27. The method of claim 26, further comprising: comparingsaid spectral curves to stored spectral curves obtained from priorsimulations; and determining amount of misalignment from result of saidcomparing.
 28. The method of claim 26, wherein said spectral curves isspectral curves of reflectivity and said calculating spectral curvescomprises analyzing a reflected light from said exposing of saidoverlaid features with a wavelength dispersive detector.
 29. The methodof claim 26, wherein said spectroscopic observations comprises measuringphase and amplitude of a reflected light from said overlaid features.30. The method of claim 29, further comprising measuring polarization ofsaid reflected light.
 31. The method of claim 26, wherein saidsimulation comprises: exposing overlaid features with differingmisalignment; and varying wavelengths or frequencies of said exposure.32. The method of claim 26, wherein said making spectroscopicobservations is performed by using a spectroscopic scatterometer. 33.The method of claim 32, wherein said spectroscopic scatterometer is of areflectometer type or an ellipsometer type.
 34. A method of measuringoverlay misalignment for features sizes less than one micron,comprising: forming a first pattern on a surface of a substrate; forminga second pattern on the surface, said second pattern superimposing saidfirst pattern to form a structure; illuminating said structure;detecting an interference pattern from said illuminated structure; andcalculating a spectrographic response corresponding to said interferencepattern.
 35. The method of claim 34, further comprising: analyzing saidspectrographic response with stored spectral curves in a library todetermine a misalignment of said first pattern and said second pattern.36. The method of claim 35, further comprising: modeling said firstpattern and said second pattern by simulation to obtain said storedspectral curves.
 37. The method of claim 35, further comprising creatingmeasured spectral curves from said spectrographic response, andcomparing said measured spectral curves with said stored spectralcurves.
 38. The method of claim 34, wherein said illuminating anddetecting are performed by a spectroscopic scatterometer.
 39. The methodof claim 38, wherein said spectroscopic scatterometer comprises areflectometer or an ellipsometer.
 40. The method of claim 34, whereinsaid illuminating comprises illuminating said structure with a broadbandlight.
 41. The method of claim 34, wherein said interference patterncomprises at least one of amplitude, phase and polarization information.42. A method for providing a quantitative measurement of misalignment byprocessing spectroscopic response corresponding to a detectedinterference pattern using broadband illumination.
 43. The method ofclaim 42, further comprising a library of characterization of change inoverlay misalignment with change in asymmetry or phase function.
 44. Anapparatus comprising: a storage device for storing spectral curves; anda specular spectroscopic scatterometer for measuring reflection from aplurality of marks formed by two lithographic exposures and forming aperiodic structure.
 45. The apparatus of claim 44, further comprising: acomparator for comparing processed signals output from said specularspectroscopic scatterometer with said stored spectral curves to evaluatemisalignment of said two lithographic exposures.
 46. The apparatus ofclaim 44, wherein said specular spectroscopic scatterometer comprises areflectometer or an ellipsometer.
 47. An apparatus comprising: a processtool comprising an optical spectroscopic reflectometry or ellipsometrysensor, wherein said sensor detects a reflected light generated from anilluminated overlay pattern formed on a substrate.
 48. The apparatus ofclaim 47, further comprising: a library for storing simulated spectralcurves; and a controller for determining an alignment error of saidoverlay pattern by comparing a measurement result of a spectral curvedetected by the sensor with said simulated spectral curves stored in thelibrary.
 49. The apparatus of claim 47, wherein said process tool is aresist track developer.
 50. The apparatus of claim 47, wherein saidprocess tool is capable of providing direct feedback on the alignmentsystem performance of a stepper.
 51. A stand-alone overlay metrologytool for in-line metrology applications comprising an opticalspectroscopic reflectometry or ellipsometry sensor for detecting areflected light generated from an illuminated overlay pattern formed ona substrate.
 52. An apparatus comprising: a specular spectroscopicscatterometer for measuring data from a mark on a substrate; anoptimizer for generating measured response curves from the measureddata; and a quantifier for quantifying misalignment error ∘ said mark.53. The apparatus of claim 52, further comprising: a data storage forstoring a library of response curves; and a comparator for comparing themeasured response curves with the library of response curves.
 54. Theapparatus of claim 52, wherein said specular spectroscopic scatterometeris one of a reflectometer or an ellipsometer.
 55. The apparatus of claim52, wherein said specular spectroscopic scatterometer comprises: a lightsource for illuminating said mark and a detector for detecting areflected light from said substrate.
 56. The apparatus of claim 55,wherein said light source is a broadband light source.
 57. The apparatusof claim 55, wherein said detector is a wavelength dispersive detector.58. The apparatus of claim 52, wherein said data comprises at least oneof amplitude, phase and polarization measurements.
 59. The apparatus ofclaim 52, wherein said mark comprises a periodic structure.
 60. Theapparatus of claim 52, wherein said mark comprise a first patternsuperimposed on a second pattern.